Synthesis and characterization of multi-spectral luminescent nanocrystals based on NaYF4 and NaGdF4

Abstract

Multispectral luminescent nanoparticles based on hexagonal-phase (β-) NaYF4 and NaGdF4 ma- trices, doped with rare-earth (RE) ions (Yb3+ /Er3+ /Tm3+ /Eu3+ ) and transition metals (Fe3+ /Mn2+ ), were successfully synthesized via the thermal decomposition method. The structural characteri- zation by X-ray diffraction (XRD) confirmed the formation of highly crystalline, single-phase

hexagonal structures with lattice parameters consistent with the reference ICSD patterns. Fourier- transform infrared (FTIR) spectroscopy verified the effective surface capping of the nanoparticles

with oleic acid (OA) ligands. The optical properties were evaluated using ultraviolet-visible (UV-Vis) absorption and photoluminescence (PL) spectroscopy. Under near-infrared (976 nm) excitation, efficient upconversion (UC) emission was observed, mediated by Yb3+ sensitization and energy transfer to Er3+ /Tm3+ activators. Core@shell architectures exhibited a significant enhancement in UC intensity compared to core-only structures, demonstrating the effectiveness of the inert shell in suppressing surface-related quenching. Under ultraviolet (365 nm) excitation, downconversion (DC) luminescence was dominated by characteristic emissions from Eu3+ , Er3+ ,

and Tm3+ , with OA acting simultaneously as a surface ligand, sensitizer, and blue emitter. The introduction of transition metals (5% Fe3+ or Mn2+

) into the core of NaGdF4-based nanoparticles

induced distinct modifications: Mn2+ doping enhanced the relative intensity of the (110) diffrac- tion peak and promoted resonant energy transfer that intensified red emission from Er3+

. The core@shell design effectively mitigated the non-radiative quenching typically associated with Mn2+ doping in core-only systems. These findings demonstrate that transition-metal codoping, combined with core@shell engineering, provides a viable route to tune energy-transfer pathways and emission profiles in RE-doped nanostructures.